The Effects of Viscosity

Generally, particle counting is performed on both new and used (in-service) oils having ISO viscosity grades of 32 to 68, and the resulting data is often compared to baseline data from the same oil. Therefore, viscosity differences are not a factor and are not taken into account; it is constant. Target ISO cleanliness codes are set for different types of machine elements (rolling bearings, journal bearings, gears, hydraulic components, etc.) and the viscosity grade of oil used is again not a factor and thus not addressed. Therefore, for the end-user of lubricants and oil analysis user, viscosity effects are generally not an issue when discussing particle count results. But within the laboratory performing the measurements, and when relatively high viscosity lubricants are included, then viscosity does play a role in obtaining an accurate particle count measurement, and for this reason, laboratory personnel must take the viscosity of the oil into account.

Figure 1

Viscosity Effects What are some of the viscosity effects of testing in-service lubricants?

1. Incomplete dispersion. It's the lab technician's job to homogenize samples by vigorous shaking. Some technicians use mechanical shakers and initially heat the sample to lower the viscosity. In other instances, an ultrasonic bath can be used to aid in the dispersion of agglomerated particulates. All of these techniques are affected by the viscosity of the fluid. Particulate matter in the fluid located near the inside boundary wall of the sample bottle does not move much, especially when testing ISO 220 and higher viscosity oils. Viscosity is a measured resistance to flow and a 320 VG oil resists movement and mixing 10 times more than a 32 VG.

2. False particle counts due to incomplete degassing. After the sample has been vigorously shaken (regardless of whether particulate debris was fully dispersed in the process) the sample is now highly aerated. Without complete degassing, the air bubbles will be counted by an optical particle counter as if they were particles. These result in false counts and thus must be eliminated before testing. The most effective and complete degassing is performed using a vacuum. Possibly the best way to quickly degas without losing particles to the sample bottom is shown in Figure 1.

Proper mixing entrains air in the sample which interferes with particle counts.

Vacuum degassing removes air bubbles and is quicker than other methods.

Degassing is performed with the syringe upside down. The sample is then flipped over for the test, ensuring large particles have not settled out.

This avoids missing larger particles due to settling.

An alternate or supplemental method for degassing is to use an ultrasonic bath, which has the added benefit of breaking up particle agglomeration but is far less effective at degassing than vacuum. No matter which method you elect to degas your samples, the major challenge to degassing is viscosity of the fluid. It is far more difficult both to homogenize and to degas higher viscosity oil. Because laboratories must process many samples and work quickly and the homogeneity is lost if you take too long degassing, incomplete degassing is a significant viscosity effect when testing in-service lubricants.

3. False counts from flowing velocity profile variation (sidebar). Optical particle counters (OPCs) have two windows through which the light or laser beams pass to count particles. The spacing between the windows is approximately 0.8 mm (0.03 inch). The fluid flows through this space and the beam slices through roughly 1 ml of fluid every second. All OPCs are designed for the flow profile to be laminar, not turbulent. The laminar velocity profile is directly related to the viscosity, where fluid in the center flows faster and the fluid near the walls barely moves due to the boundary layer. Thin fluids, such as VG 16 calibration oils, have thin boundary layers. Viscous fluids, such as ISO VG 100 and above, contain different velocity profiles with thicker boundary layers. At the interface between velocity layers in the laminar flow profile, an OPC can experience refraction variations which can be thought of as "flutter". The more viscous the fluid, the more flutter occurs. Flutter is detected by the OPC as false counts, particularly in the small size ranges.

4. False counts from cross-contamination. This is important when using OPCs to test different in-service lubricants in the same particle counter. In this case, viscosity variations are compounded by differences in base oil compatibility and contaminating substances. To get consistent sample-to-sample results, it is necessary to completely flush out the inside of the tubing and connectors through which the sample fluid flows before reaching the OPC. The length of travel from the sample injection port to the OPC varies with design. Some are short, approximately 5 cm, while most are 15 cm or longer. High-viscosity lubricants are persistent, particularly if they carry significant amounts of water or particulate contamination. It is easier to flush out an ISO VG 32 hydraulic oil than in-service lubricants having ISO VG 220, 320 or 460. Cross-contamination of in-service lubricants or from switching between synthetic and petroleum base oils can present errors or fluctuations in the data.

5. False counts from additive effects. Dr. T. Tim Nadasdi, products technical advisor for ExxonMobil Lubricants and Specialties Company, has described various additive effects on particle counting. "Oil viscosity can have an effect on light blockage particle counts through certain insoluble additives in finished lubricants. For instance, antifoamants are normally insoluble liquids dispersed in oils. Because they are insoluble and have a different refractive index than the oil itself, they can be counted as hard particles in a particle counter. If the insoluble liquids are well dispersed, the size of the dispersed spheres can be less than four microns, which may be invisible to the particle counter. However, with higher viscosity oils, it can be difficult to disperse the insoluble liquids, leading to higher size spheres and a higher particle count."

Table 1. ASTM Blending Chart

Improving Particle Counting Diluting all oil samples before optical particle counting1 has several advantages. It eliminates most cross-contamination, eliminates viscosity effects which are significant for high-viscosity lubricants and not as significant for lower viscosity oils. It also broadens the range of measurement to cover dirty samples, addresses problems with air bubbles without losing large particles, allows for testing of dark oils, and eliminates immiscible fluid counts and false counts from additive issues.

If an ISO 220 cSt gear oil is mixed in an equal ratio with 3 cSt kerosene, what is the resulting viscosity? If you guessed that the result would be in the middle, or about 111 cSt, you are incorrect! According to the ASTM viscosity blending chart, the 50:50 mixture will result in a fluid that is only 15 cSt in viscosity (all at 40°C). That is a 93 percent viscosity drop!

The ASTM blending chart (Table 1) can be used to find the resulting viscosity for various 50:50 (or other ratio) blends of different ISO-grade oils with kerosene.

It can be seen that the lower viscosity fluid dominates the blend viscosity. Table 1 and associated Figure 2 illustrate that when diluting oil samples 50:50, the resulting viscosity is in the range of 8 to 20 cSt, compared to the starting viscosity range, 32 to 680 cSt of the oil.

Kerosene, diesel fuel, lamp oil, heptane, particle counter "water masking solvent", or other solvents will all have a similar effect. Regardless of the starting oil viscosity, when a 50:50 blend is created with a light solvent, the resulting mixture will be approximately 10 cSt at 40°C.

Figure 2. Viscosity After Dilution

Diluting Therefore, dilution is strongly recommended when testing samples with OPCs. Dilution with solvent reduces the viscosity of the lubricants being tested to less than 10 percent of the initial value in centistokes, which allows the sample to be well mixed. It permits particle agglomerates to be broken up by agitation and distributes contamination well within the sample. Dilution also allows highly contaminated samples to be measured with accurate particle counts and well-defined size distribution. The diluted sample will flow past the laser sensor with the least shear, allowing accurate and repeatable measurements.

When diluting, one may elect to use a water-masking solvent to eliminate the false counts from emulsified water-in-oil. This also works well with water-glycol fluids. Water masking may be performed on all samples or as an extra step (second dilution) (Table 2).

It is crucial to account for all the particles when performing diluted sample testing, including the counts contributed by the solvent. It is recommended to use software to guide the operator through the process, beginning with determining the contamination level for each solvent and tracking the fractional dilution by weight. Therefore, the software subtracts counts attributed to the solvent fraction, and reports only the counts and size distribution for the in-service fluid.

Does viscosity affect optical particle counting of in-service lubricants? Most definitely! Dispersion, degassing, velocity profile, cross-contamination and additives may all produce viscosity effects. What can be done about this? Include dilution in the laboratory procedure. Dilution counteracts each of the viscosity effects mentioned above and allows fluids up to 1,000 cSt at 40°C to be tested.